In the field of infrared detectors, it is known to use devices arranged in matrix form, and suitable for operating at ambient temperature, that is, they are not required to be cooled to very low temperatures, contrary to the detection devices called “quantum detectors,” which require operation at a very low temperature, typically that of liquid nitrogen.
These uncooled detectors conventionally use the variation in a physical quantity of an appropriate material, as a function of temperature, in the neighborhood of 300 K. In the case of the bolometric detectors, this physical quantity is electrical resistivity.
Such an uncooled detector generally associates:                means for absorbing the infrared radiation and for converting it to heat,        means for thermally insulating the detector, so as to enable it to be heated under the action of the infrared radiation,        thermometer means which, in the context of a bolometric detector, use a resistive element,        and means for reading the electrical signals supplied by the thermometer means.        
Detectors for infrared imaging are conventionally made in the form of a matrix of elementary detectors in one or two dimensions, said matrix being formed in a manner called “monolithic” or transferred to a substrate generally made from silicon, in which means for sequentially addressing the elementary detectors are constituted, and means for the electrical excitation and preprocessing of the electrical signals generated by these elementary detectors. This substrate and the integrated means are commonly designated by the term of “read circuit”.
To obtain a scene by means of this detector, the scene is projected through an optic adapted to the matrix of elementary detectors, and timed electrical stimuli are applied via the read circuit to each of the elementary detectors, or to each row of such detectors, in order to obtain an electrical signal constituting the image of the temperature reached by each of said elementary detectors. This signal is processed in a more or less elaborate manner by the read circuit, and then optionally by an electronic device outside of the package, in order to generate the thermal image of the scene observed.
The essential difficulty in the use of bolometric detectors resides in the very small relative variation in their electrical resistance, representative of the local temperature variations of an observed scene, compared to the mean value of these resistances.
In fact, the physical laws of thermal emission in the infrared of the scene observed between 8 and 14 μm (corresponding to the transparency band of the earth's atmosphere in which bolometric detectors are commonly used) lead to a differential power dP of about 50 μW/cm2 at the detector focal plane when the scene temperature varies by 1 K around 300 K. The determination of this value is easily within the scope of a person skilled in the art, by applying the abovementioned physical laws.
This estimation is valid for an optic with an aperture of f/1, a high transmission between the scene and the detector, and when the detector only receives a negligible quantity of energy outside the specified wavelength band, for example and typically if the package is provided with a window that is transparent in this interval and opaque on either side of the indicated limits.
As a corollary, the temperature variation dT of a bolometer at thermal equilibrium related to the infrared power dP absorbed on its surface S is given by the expression:dT=Rth.dP,where Rth is a thermal resistance between the sensitive part of the bolometer, which is heated under the effect of the infrared radiation, and the isothermal substrate supporting it.
Thus, for a bolometer having typical dimensions of about 30 μm×30 μm, which represents an area of 9.10−6 cm2, the typical thermal resistance according to the prior art is about 20 to 60 MK/W, which leads to a temperature rise of the bolometer of about 0.01 K to 0.03 K when the temperature of the scene element viewed by this bolometer varies by 1 K.
If Rb denotes the electrical resistance viewed between the two current intake poles at the bolometeric sensitive material, the resulting resistance variation dRb is expressed by the expression:dRb=TCR.dTwhere TCR is a coefficient of relative variation of resistance of the material constituting the sensitive part of the bolometer in the neighborhood of the operating temperature, which is conventionally close to −2% per K for commonly used materials in this field (vanadium oxides, amorphous silicon). In consequence, the relative variation in resistance dR/R resulting from a differential of 1 K in the scene is about 0.02 to 0.06%, or 2.10−4 to 6.10−4/K.
Today, however, the thermal imaging resolutions demanded are much better than 1 K and typically 0.05 K, or even less. Such results can be obtained by the preparation of structures having very high thermal resistances Rth involving the use of sophisticated techniques. However, the need remains to measure infinitesimal relative variations in resistance and typically, as previously indicated, of about a few 10−6, to resolve the time-space temperature variations of a few tens of milliKelvin.
In order to clarify the difficulty of using such a small variation, FIG. 1 shows a schematic of a read circuit of a resistive bolometer 12 having a resistance Rb, subject to an infrared radiation and connected at one of its terminals to a predefined constant bias voltage VDDA. The read circuit comprises an integrator 10 formed from:                an operational amplifier 14 whereof the non-reversing input (+) is set at a predefined constant voltage Vbus;        a capacitor 16, having a predefined capacitance Cint, connected between the reversing input (−) of the amplifier 14 and the output thereof;        a zero reset switch 18 connected in parallel from the capacitor 16 and controllable by means of the Reset signal.        
The read circuit further comprises:                a first read switch 20 controllable by means of a “Select” signal and connected to the reversing input (−) of the operational amplifier;        an MOS injection transistor 22, whereof the grid is set at a predefined constant voltage GFID, whereof the source is connected to another terminal of the bolometer 12, and whereof the drain is connected to the other terminal of the first selection switch 20; and        a data processing unit 23, connected at the output of the operational amplifier 14, and determining, according to the voltage Vout at the output thereof, the variation in resistance of the bolometer 12 caused by the infrared radiation received by said bolometer, and hence the infrared radiation.        
At the start of a read cycle of the bolometer 12, the zero reset switch 18, which is at the closed state after a discharge cycle of the capacitor 16, is switched to the open state by setting the Reset signal at an appropriate value. The first read switch 20, which is in the opened state, is switched to the closed state by adjusting the “Select” signal. The current flowing through the bolometer 12 is then integrated by the capacitor 16. When a predefined integration time ΔTint has elapsed since the start of the read cycle, the first read switch 20 is switched to its open state. The voltage Vout at the output of this integrator, the image of the resistance Rb of the bolometer, is then given by the expression:
      V    out    =                    V        bolo                    R        b              ×                  Δ        ⁢                                  ⁢                  T          int                            C        int            where Vbolo is the voltage at the terminals of the bolometer 12, by assuming, for simplification, that Rb varies only slightly during the integration time Tint.
Thus, a matrix of N resistances (bolometers) could be read by this principle using the simultaneous integration (by means of N integrators) or sequential integration (on an integrator placed at the end of the line or end of the column, or even a single integrator for the matrix).
When the matrix thus prepared is lit by the projection of an infrared scene, Vout will display spatial variations (issuing from each bolometer) that are representative of the scene. It may be recalled that the voltage Vout as previously expressed consists very largely of a constant portion from one detector to the next (signal called “common mode”), which is therefore of no interest in terms of imaging. Only the infinitesimal variations in Vout associated with the local differences (between one bolometer and the other) and temporal variation (the scene varies over time) in the optical power received constitute the useful signal of the scene observed.
In fact, the limitations inherent in microelectronic circuits in terms of voltage (a few volts), the accessible and controllable values of the bolometric resistances Rb (a few tens to a few hundred kOhms) and the need to use sufficiently short integration times, would lead to the use of very high capacitances Cint, in any case incompatible with the area available in each elementary point or detection pixel (about that of a bolometer), and even in practice incompatible with a transfer of this capacitance to the exterior of the surface of the read circuit corresponding to the sensitive matrix, where the area is nevertheless not limited. It is therefore necessary to install read modes which limit the current to be integrated to levels compatible with reasonably obtainable capacitances.
Furthermore, due to the existence of the thermal coupling between the substrate and the bolometer, the thermal variations undergone by the substrate are transferred to the bolometer. Since common bolometers have a very high sensitivity to such variations, the consequence thereof is that the useful output signal is disturbed by this background component, which is harmful to the quality of detection of the infrared radiation.
To overcome these drawbacks, a first resistive structure has been proposed designed to inhibit the common mode current, called “reference” current described in the document “Performance of 320×240 Uncooled Bolometer-type Infrared Focal Plane Arrays” by Yutaka Tanake et al., Proc. SPIE, vol 5074.
The principle of a reference resistive structure is to associate the resistive bolometer 12 in FIG. 1 with a second identical resistive bolometer, polarized and connected to the substrate identically to the first bolometer. This second bolometer is further arranged in order to be essentially insensitive to the stream from the scene, typically by an opaque metal membrane. The first and second resistive bolometers are also associated so that the current flowing through the second bolometer is subtracted from the current flowing to the first bolometer and it is this current difference that is used by the read circuit.
In order to distinguish between the functions of these two bolometers, the expression “imaging” bolometer is used for the first bolometer, and the expression “reference” bolometer is used for the second bolometer, even though in certain applications, thermometry for example, an image is not necessarily formed, but a temperature measurement for example.
A reference structure 24 is shown schematically in FIG. 2A, which resumes the elements in FIG. 1, with which a circuit called “reference” circuit 24 is associated. The reference circuit 24 comprises a reference bolometer 26, an MOS polarization transistor 28 and a second read switch 30, respectively substantially identical to the imaging bolometer 12, the MOS injection transistor 22 and the first read switch 20.
The elements 26, 28 and 30 are also polarized and arranged in the same manner as the elements 12, 22 and 20, with the only difference that the reference bolometer 26 is provided with an opaque metal membrane 32 protecting it from the radiation issuing from the scene.
The resistive reference structure also comprises a current mirror 34, whereof one input branch is connected to a terminal A of the second read switch 30, and whereof the other input branch is connected to a terminal B of the first read switch 20. This current mirror 34 substantially reproduces the current i2 flowing through the reference bolometer 26 at the terminal B.
The use of current mirrors serves to have a single reference structure per line, the combination of these structures being disposed along a reference “column” for a matrix detector. Current mirrors are structures known to a person skilled in the art. They serve in general to copy a reference current in a distant structure, and in particular, they allow the distribution of this reference current in a multitude of circuitry elements, independently of their resistive charge.
Thus, the current i2 flowing through the reference bolometer is substantially equal to the common mode current, and the reference bolometer is subject to the same thermal variations from the substrate as the imaging bolometer. The difference i1-i2 between the current i1 flowing through the imaging bolometer and the current i2 flowing through the reference bolometer is accordingly substantially free of the disturbances, which are the common mode current and the component associated with the thermal variations of the substrate. This current difference i1-i2 therefore corresponds substantially to the current induced by the variation in resistance of the imaging bolometer 12 owing to its heating by the infrared radiation issuing from the scene.
However, a resistive reference structure is technically difficult to produce. In fact, to obtain a satisfactory operation thereof, it is necessary for the metal membrane 32 protecting the reference bolometer to be totally opaque to the stream issuing from the scene, while being thermally insulated from the other elements of the structure to avoid any thermal disturbance on the reference bolometer. It is easy to see that such a membrane is difficult to design and to produce.
To overcome the drawbacks mentioned above, a second resistive structure for inhibiting the common mode current has also been proposed, called “compensation” resistive structure, described in the document “Uncooled amorphous silicon enhancement for 25 μm pixel pitch achievement” E Mottin et al, Proc. SPIE, Technology and Application XXVIII, Vol 4820.
FIG. 2B schematically shows this compensation structure 52, which comprises a bolometer 50 typically constructed using the same material as the active bolometer 12, but essentially insensitive to the incident radiation due to a thermal resistance which is very weak by construction compared to the substrate, and optionally further provided with an optical shield, and also a transistor 54 for polarization of the bolometer 50. The bolometer called “thermalized” 50 is connected at one of its terminals to a fixed voltage source VSK, and at the other terminal to the source of a transistor 54 whereof the grid is raised to a fixed potential GSK and whereof the drain is connected to the reversing input (−) of the amplifier 14.
The value of the resistance 50 and the polarizations are set so as to produce a common mode current I3 with an intensity comparable to the current I1, which is subtracted from the current I1 at the summation point of the integrator 10, which therefore integrates the current I1-I3.
This structure is effective in terms of common mode rejection if the thermal resistance of the bolometer 50 is very low compared to that of the imaging bolometer 12, typically by a factor of at least about 103, because if not, undesirable contrasts are formed, detrimental to the quality of the image, which are not representative of the scene, particularly when a very warm zone of the scene forms an image on these structures. This result of high thermal conduction may be reached for example by constructing the bolometric elements 50 directly in contact with the substrate.
However, such a construction raises problems that are difficult to control in terms of the available planes of the structures during the technological assembly, and in practice, one is forced to form the bolometers 50 at the same level as the sensitive membranes of the bolometers 12. It follows that a non-zero thermal resistance normally subsists between the element 50 and the substrate, unless complex technological measures are taken, which are detrimental to the manufacturing output and the cost of the detectors thus prepared.
In this case of the application of a reference structure, which is at least useful during the use of a compensation structure, it is therefore necessary to provide an opaque shield of optical desensitization on these common mode rejection structures, whereas the substantial complexification that the preparation of such a membrane implies necessarily entails an additional cost, due to the additional steps in manufacture and the necessarily lower production yield.